Theoretical Studies on the Photoelectron and Absorption Spectra of MnO₄^–and TcO₄^–

Abstract

The tetraoxo pertechnetate anion (TcO4(-)) is of great interest for nuclear waste management and radiopharmceuticals. To elucidate its electronic structure and to compare with that of its lighter congener MnO4(-), the photoelectron and electronic absorption spectra of MnO4(-) and TcO4(-) are investigated with density functional theory (DFT) and ab initio wave function theory (WFT). The vertical electron detachment energies (VDEs) of MnO4(-) obtained with the CR-EOM-CCSD(T) method are in good agreement with the lowest two experimental VDEs; the differences are less than 0.1 eV, representing a significant improvement over the IP-EOM-CCSD(T) result in the literature. Combining our CCSD(T) and CR-EOM-CCSD(T) results, the first five VDEs of TcO4(-) are estimated between 5 and 10 eV with an estimated accuracy of about ±0.2 eV. The vertical excitation energies are determined by using TD-DFT, CR-EOM-CCSD(T), and RAS-PT2 methods. The excitation energies and the assignments of the spectra are analyzed and partly improved. They are compared with reported SAC-CI results and available experimental data. Both dynamic and nondynamic electron correlations are important in the ground and excited states of MnO4(-) and TcO4(-). Nondynamical correlations are particularly relevant in TcO4(-) for reliable prediction of excitation energies. In TcO4(-) one Rydberg state interlaces but does not mix with the valence excited states, and it disappears in the condensed phase.

Modeling the absorption spectrum of the permanganate ion in vacuum and in aqueous solution

“…Transition metal complexes can generally be expected to display multireference character, and in such cases both dynamical and static correlation must be modeled accurately with multireference methods, such as complete active space second-order perturbation theory (CASPT2). 14,20 or a somewhat more pronounced red-shift of −0.13 eV, 10,22 in clear contrast to experiment. It is presently not known whether the discrepancies are caused by inadequate electronic structure methods, solvent models, or both.…”

“…The CAS(14,12)-srPBE calculation based on the vacuum geometry gives the values that can be compared to the accurate results based on RAS(24,17)PT2. 20 In terms of excitation energies both methods agree to within 0.1 eV for the first two transitions, and 0.2 eV for the third transition.…”

“…This is 0.04 eV higher than the energy obtained using the geometry-optimized The spectrum in figure 1 can be interpreted on the basis of the individual states given in 20 With such a small energy-gap, the two states are expected to mix when the tetrahedral symmetry is lifted. This is indeed the case, but among the lowest six states, the 1 1 T 1 and 1 1 T 2 multiplets can still be discerned (see table 2).…”

“…34 For these higherlying excitations (close to the ionization limit) the inclusion of Rydberg basis sets can be of importance, although it had only little effect in ref. 20. CAS-srDFT has previously been noted to be more sensitive than e.g.…”

Applications of Time-Dependent and Time-Independent Density Functional Theory to Electronic Transitions in Tetrahedral d0 Metal Oxides

“…We compare in Table 6 multiplet energies based on RSCF-CV-DFT and TDDFT with those obtained from the high-level wave function methods 60 − and TcO 4 is dominated by the t 1 → 2e orbital transition, which gives rise to the 1T 1 and 1T 2 multiplets. All methods find the order 1T 1 < 1T 2 , which is in agreement with the experiment.…”